The present invention relates to power amplifiers, and in particular to an RF power amplifier with a distributed bias circuit.
In the field of bipolar radiofrequency (RF) power amplifiers, the biasing of the amplifying transistor(s) is crucial in determining the class of operation and the linearity of amplifier response. Various conventional biasing schemes for such amplifiers are known. These biasing schemes must typically balance competing considerations such as linearity, noise figure, distortion and power consumption.
It is well known that a bipolar transistor is subject to thermal instability and runaway if it is not properly biased. Thus, in power amplifiers, amplifying transistors are commonly ballasted for thermal stability by applying degeneration (ballast resistors) in the base and/or emitter DC current paths.
While base ballast resistors provide thermal stability, they can adversely affect amplifier performance. A base ballast resistor must be relatively large to provide adequate temperature compensation. As input signal power is increased, the large resistance in the base DC path reduces the amount of power available at the amplifier output. As a result, a base-ballasted amplifier exhibits non-linear response.
A distributed base bias circuit is described in U.S. Pat. No. 5,150,076, entitled xe2x80x9cEmitter-Grounded Amplifier Circuit With Bias Circuit.xe2x80x9d This active bias scheme eliminates some of the disadvantages of base ballast resistors. However, the scheme described in that patent exhibits undesirable thermal characteristics. Moreover, the device""s sensitivity to supply voltage fluctuations requires a relatively high supply voltage to be used, making the amplifier less suitable for portable applications such as cellular telephones.
Therefore, a need has arisen for an RF power amplifier that addresses the disadvantages and deficiencies of the prior art. In particular, a need has arisen for an RF power amplifier with a robust, thermally compensated biasing scheme.
Accordingly, an improved amplifier circuit is disclosed. In one embodiment, the amplifier circuit includes an amplifier transistor that has a base terminal connected to receive an input signal. The amplifier transistor amplifies the input signal to generate an output signal. A reference voltage source generates a reference voltage at a reference voltage output node. A local bias circuit provides a bias voltage to the base terminal of the amplifier transistor. The local bias circuit includes a first transistor that has an emitter terminal coupled to the reference voltage output node, a collector terminal coupled to a supply voltage node, and a base terminal connected to the collector terminal. The local bias circuit also includes a second transistor that has a base terminal coupled to the base terminal of the first transistor, a collector terminal coupled to the supply voltage node, and an emitter terminal coupled to the base terminal of the amplifier transistor.
An advantage of the present invention is that a robust, thermally compensated bias voltage is provided to any number of amplifier cells. Another advantage of the present invention is that the bias circuit is ballasted to prevent thermal runaway in any one of the amplifier cells.